In recent years, organic electroluminescent elements composed of organic materials (hereinafter, also referred to as organic EL elements) have promise as inexpensive large-area full-color display devices of solid emission types and writing light-source arrays and thus have been under extensive research and development.
Organic EL elements are full solid state elements of thin film types, each including a single or multiple organic functional layers having a thickness of about 0.1 μm and containing an organic luminescent material between an anode film and a cathode film. When a relatively low voltage of about 2 to 20 V is applied to such an organic EL element, electrons are injected from the cathode into the organic functional layer(s) while holes are injected from the anode into the organic functional layer(s). The electrons and holes recombine in the luminous layer and the resulting excitons are returned to the ground state. At this time, energy is emitted as light to perform light emission. Such devices are expected as next-generation flat displays and illuminating devices.
Organic EL elements by phospholuminescent emission, which has been recently discovered, can emit light at a luminous efficiency about four times higher than that of conventional fluorescence, in principle. Material development for the elements and research on the configuration of organic functional layer(s) and electrodes are under study all over the world. In particular, application to lighting systems comes under review as a measure against global warming because lighting systems account for a large percentage of energy consumed by human beings. Improvements in performance and reductions in costs have been extensively attempted toward the practical use of white light-emitting panels in place of conventional lighting systems.
Requirements for white light-emitting panels for lighting are high light emission efficiency and prolonged service life. Unfortunately, the service life of the panels is still insufficient compared to fluorescent lamps and white light LEDs.
Although some blue phosphorescent compounds having high luminous efficiency have been discovered, these are still unsatisfactory in view of compatibility with wet coating processes, service life, and color purity at the present stage.
A countermeasure to solve these problems is use of “quantum dots”, which are inorganic luminous substances, as luminous materials.
Quantum dots have sharp luminescent spectrum characteristics. They, which are inorganic compounds, further have advantageous characteristics suitable for wet coating processes, such as high durability and high dispersion in a variety of solvents.
For example, Patent Literature 1 discloses white light emission achieved by a layer containing a quantum dot formed on a light-emitting side of a luminescent element in which emission involves photoexcitation by down conversion for compensation for the color of the light emitted from the luminous layer. Unfortunately, the emission lifetime, depending on characteristics of materials in the luminous layer, is still insufficient.
Patent Literature 2 discloses white light emission by a hole transport layer containing two types of quantum dots or a fluorescent polymer material. Unfortunately, this technology can contain a limited amount of quantum dot material causing concentration quenching and thus exhibits insufficient brightness efficiency. A possible technique to achieve high-quality white light emission, i.e., white light emission having high color rendering properties is addition of a variety of quantum dots having different particle sizes. Since large amounts of various quantum dots cannot be added for the reason described above, the element exhibits unsatisfactory color rendering properties and fluctuated color indicating a color shift during a continuous drive.
Some organic EL elements that have been disclosed in recent years have a plurality of luminous layers and charge generating layers (hereinafter also referred to as CGLs) disposed between the luminous layers to improve the luminous efficiency.
For example, Patent Literature 3 discloses a structure including a 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (hereinafter referred to as HATCN or HAT) layer in place of a molybdenum layer as part of a charge generating layer. Patent Literature 4 discloses a charge generating layer primarily containing an organic compound or its metal complex. Unfortunately, in both structures disclosed in Patent Literatures 3 and 4, the multiple layer configurations inevitably require high driving voltages and thus low power efficiency. In addition, the half-life luminance of organic EL elements should be further improved.